1. Field of the Invention
The present invention relates to an optical device and a method for measuring the rotation and/or the rotational vibrations of an object.
More particularly, the invention relates to an optical device and a method for measuring the rotations or rotational vibrations of an object, wherein the rotations are of low angular amplitude at least of the order of 0.1 micro-radian) and the vibration frequencies thereof can reach several hundreds of Hertz.
2. Description of the Related Art
Indeed, it is important to be able to measure the very low amplitude rotations and/or rotational vibrations of certain objects. For example, on a very long laser chain, the rotational vibrations of components coming in the optical alignment may produce amplified vibrations of the beams and make the alignment very difficult to get. To identify the source of the mechanical vibrations and to remedy them, it is important to measure the amplitude of the rotational vibrations and, if needed, to analyze the frequency spectrum thereof.
Various categories of devices permit to more or less directly obtain a measurement of rotation and/or rotational vibrations of an object.
A first category of devices uses several accelerometers that provide translation measurements, from which is deduced an indirect rotation measurement. A first method is based on the numerical adjustment of the parameters of a mechanical model to obtain an optimized simulation of the translational vibrations compared to the measurements. The thus-determined mechanical model is then used to calculate an estimate of the rotations. Although the mechanical models are often simplified with respect to the reality, the numerical adjustments need relatively long and costly calculation times. A second method uses the translational vibration measurements of several accelerometers whose readings are synchronized. The differential measurement of the displacements divided by the distance between the sensors permits to deduce the angular vibrations as a function of time. However, the accuracy of measurement is related to the spacing between the sensors. This minimal spacing required is difficult to implement for small size objects. Moreover, the sensors can be inopportunely placed at positions that are insensitive to certain natural modes of vibration of the object (the vibration nodes).
A second category of rotation measuring device is based on the use of an autocollimation optical method illustrated in FIG. 1. An autocollimator telescope 1 comprises a source 4 and a position sensor 7. The collimated beam 8 of the source is directed at normal incidence to a reflecting plane face 3 of the object 2 or to a flat mirror fastened to the object to be measured.
In a known manner, when the reflecting face 3 undergoes a rotation of an angle θ, the reflected beam 9 undergoes an angular deviation 2θ. A position sensor 7 placed at the telescope focal point permits to measure the deviation of the reflected beam projected on the sensor 11, induced by a rotation of the object with respect to an initial position of the projected reflected beam 10. The distance from the telescope to the object being known, the measurement of the reflected beam displacement permits to directly calculate the corresponding rotation angle of the object. The measurement accuracy is a function of the distance between the object and the autocollimator telescope.
However, the rotation measurement obtained by an autocollimator telescope is not an absolute measurement of the object rotations, but only a relative measurement of the rotations between the object and the telescope. Indeed, it is not possible to make the difference between the movements of the telescope and those of the object. Ensuring the stability of the telescope is thus a major stake, which is difficult or even impossible to realize for the measurement of stable objects whose movements are low in amplitude or when it is impossible to isolate the telescope from the ambient vibrations.